This is an observational 48 week study of lean body mass, appetite, functional performance, and systemic markers of inflammation during highly active antiretroviral therapy (HAART) in subjects co-enrolled in ACTG antiretroviral studies. The study will determine if changes in body composition are related to changes in HIV-1 RNA levels and whether the magnitude of these changes in body composition is quantitatively related to (or predictive of) risks of AIDS-defining complications. One hundred subjects will be enrolled into each cohort: one cohort at a lower body mass (BMI<23) and one cohort at a higher body mass (BMI>=23 to 28 Kg(m2) to assure that the entire spectrum of subjects with various body compositions are proportionally included in the study.